1. Field of the Invention
The invention relates to a nonlinear frequency converter apparatus particularly for diode-pumped solid state lasers, and more particularly to an automatic phase matching adjustment method and apparatus for incident and converted wavelength components.
2. Discussion of the Related Art
Diode pumped solid state lasers are efficient, compact and reliable sources of high beam quality optical radiation. The group of solid state lasers includes most commonly the neodymium laser, and also ruby lasers, but there are many others. Triply ionized neodymium is the active material of neodymium lasers. In a crystal, the neodymium is a substitutional dopant (most commonly for yttrium). Neodymium may also be incorporated into a glassy matrix. Neodymium may further form part of a crystal, such as with neodymium pentaphosphate NdP5O14. The most common host for neodymium is yttrium aluminum garnet (YAG), or Y3AI5O12. Other common neodymium hosts include yttrium lithium fluoride (YLF), or YLiF4; gadolinium scandium gallium garnet (GSGG), or Gd3Sc2Ga3O12; yttrium aluminate (YALO or YAP), or YALO3; and yttrium vanadate (YVO), or YVO4. Neodymium may also be hosted by phosphate and silicate glasses. Some more recently discovered neodymium host materials holding promise in the solid state laser field include gadolinium vanadate (GdVO), or GdVO4; and yttrium vanadate (YVO), or YVO4. Ytterbium (Yb) is also being doped into such crystals as YAG, YLF and YVO. Gadolinium vanadate crystals may also be doped with thulium (Tm) or thulium-holmium (Tm,Ho), rather than with neodymium. Titanium doped sapphire (Ti:Al2O3) and erbium doped YAG (Er:YAG) are also coming into vogue in the solid state laser field.
The principal wavelengths of lasing action for most solid state lasers is in the infrared (IR) spectral range. However, it is desirable to convert solid state lasers to lase in the ultraviolet (UV) spectral range. This frequency conversion is achieved with high efficiency by means of nonlinear optical conversion using nonlinear optical crystals. These crystals are normally arranged in the laser setup within the laser resonator for CW systems and outside the laser resonator for pulsed systems.
Commonly employed nonlinear conversion processes are harmonic generation, such as second, third and fourth harmonic generation (SHG, THG and FHG, respectively), and sum frequency generation (SFG). Other techniques include Raman shifting, sum and difference frequency mixing and parametric conversion. Harmonic generators may be packaged with the laser. Other techniques such as the above-mentioned ones are normally done using separate accessories. Many nonlinear optical crystals are available for doubling of the light frequency of solid state lasers thus converting the light into the visible range. However, efficient quadrupling and quintupling of laser radiation present significant challenges due to a very limited selection of nonlinear crystals and a necessity for special operating conditions for efficient and long-lasting operation.
Very few nonlinear crystals are available for nonlinear conversions of solid state laser light below 300 nanometers (nm) due to their transparency, non-linear coefficients and adequate birefringence. Of these, beta barium borate (BBO), or _xe2x80x94BaB2O4, lithium borate (LBO), or LiB3O5, and Cesium Lithium Borate (CLBO), or CsLiB6O10, each allow efficient conversion to the shortest wavelengths.
Laser systems using frequency conversion are being used in increasingly demanding industrial, medical and scientific applications. For example, it is desired for micro-machining of various materials in electronics and medical device manufacturing to have a laser with a short output wavelength and that has high stability and may be operated over a long term in a hands-off capacity. Minimizing the number and/or duration between scheduled maintenance of the laser system advantageously results in a reduction of running cost and an increase of production throughput. Conventional laser frequency converters require periodic re-adjustment of the phase-matching angle of the crystal during which time production is interrupted. For maximizing conversion efficiency, it is desired that a phase matching condition be continuously fulfilled, wherein the phase velocities for both an incident and a converted wavelength are made equal.
A common method utilizes a natural birefringency of the nonlinear optical crystal. When this method is applied, for example, to second harmonic generation, one of the waves is ordinary and the other is extraordinary. The direction of propagation of the two waves in the crystal is chosen so that refractive indices for the two waves are equal to each other (see e.g. J. E. Midwinter, J. Warner, Brit. J. Applied Physics, v. 16, p. 1135 (1965); and G. C. Bhar, D. C. Hanna, B. Luther-Davies, R. C. Smith, Optics Communications, v.6, p.323 (1972), each of which is hereby incorporated by reference).
This direction is characterized by a certain angle, called the xe2x80x9cphase-matchingxe2x80x9d angle, between the direction of propagation of the waves and an optical axis of the crystal. It is desired to have an accuracy of maintaining the phase-matching angle that is substantially better than 1 mrad, such as 0.1 mrad. Additionally, since the ordinary and extra-ordinary refractive indexes are temperature dependent, the phase-matching angle is a function of crystal temperature. Therefore, it is desired to maintain the crystal at a constant temperature, in addition to maintaining the direction of the two waves at the phase-matching angle. Likewise, the phase matching condition can be attained by either varying the angle at constant temperature, or varying the crystal temperature at a fixed beam propagation angle with respect to the crystallographic axes.
Therefore, it is desired to have a frequency converter undergo periodic re-alignment of the angle of the non-linear optical crystal angle and/or the temperature of the crystal, in order to maintain an optimal conversion efficiency. It is recognized herein that a possible way of reducing the frequency of realignments is to provide a mechanically highly stable beam direction with respect to the optical axis of the crystal, and to stabilize the temperature of the crystal in absolute terms, e.g., by means of high-accuracy temperature controller. This approach may be difficult to implement in practice, particularly since the local temperature inside the crystal may be different from the temperature of the crystal holder, which would typically be what is stabilized by the temperature controller. The reason for this difference is most commonly local heating due to absorption of the laser beam at defect and impurity sites within the crystal. Since the amount of heat depends on the laser power and the condition of the crystal, such local temperature variations lead to instability and a hysteresis of the output power of the converter.
Therefore, it is desired to have means for adjusting the phase-matching in the crystal based on parameters of the converted beam, in a closed-loop arrangement, rather than relying on the stability of environmental parameters. We can propose several approaches. For example, the power of the converted beam can be optimized by means of some algorithm that scans a range of temperatures/angles to determine an optimum position, The main problem here is that based on a single measurement of the output power alone, the direction of adjustment of the phase-matching angle or temperature in order to restore perfect alignment is not determined. The algorithm might includes varying the beam angle (or crystal temperature) and simultaneously monitoring changes in the output power, for determining the optimum angle (or temperature). A disadvantage of this algorithm is that it leads to variations of the power of converted beam, thus rendering on-line adjustment using this technique undesirable.
U.S. Pat. No. 3,962,576, which is hereby incorporated by reference, discloses an automatic phase matching apparatus and method which is illustrated at FIG. 1. As shown in FIG. 1, the crystal is mounted in a cell that is rotatable. A portion of an incident beam passing through the crystal is reflected by a quartz plate. The reflected light is received at a pair of photoelectric detectors connected to an operational amplifier. The photoelectric detectors are disposed side-by-side in the beam path of the reflected beam. When the output of the two detectors is not equal, the op amp signals a driving unit to rotate the crystal.
The position-sensitive detector in the ""576 patent measures a shift of a center of a converted beam as the wavelength of the incident beam changes. Such shift occurs because of wavelength dispersion of the refractive indexes of the nonlinear crystal. Thus, different wavelengths have different phase-matching angles at a given temperature. Disadvantages of the system shown at FIG. 1 include: (1) changes in the direction of the incident beam lead to misalignment of the entire apparatus, since the converted beam at optimal phase-matching condition is no longer centered on the detector; and (2) rotation of the nonlinear crystal leads to a parallel shift of the transmitted beam which causes misalignment of the incident beam, thus causing the same problem as in (1). A possible solution for this second problem could be immersing the crystal into optically transparent and index-matched fluid. However, such fluid may not be available or desirable for every crystal or wavelength.
It is recognized herein that it is wrong to assume that the fundamental harmonic component remains fixed at the optimum angle, so that adjusting the crystal axis orientation to move the total output beam to center is assumed to move the secondary harmonic to center. This may not be the case, since if the fundamental harmonic is off center and the secondary harmonic is off center in the opposite direction, then the total beam may be centered, or appear to have an optimized orientation relative to input beam direction, while the secondary harmonic is off center.
FIGS. 2a-2b illustrate what is recognized herein as a problem with the above method and apparatus, e.g., as set forth in the ""576 patent and shown herein at FIG. 1. At FIG. 2a, a perfect phase matching condition is met, wherein an unconverted portion of the incident beam and the converted beam are each aligned along the phase matching direction, wherein the respective centers of their intensity distributions, x1 and x2, are determined to be individually aligned to produce an overall intensity maximum centered at x1=X2.
At FIG. 2b, a de-tuned phase matching condition is illustrated, wherein the two beams are not aligned along the phase matching direction, and instead are misaligned in opposite directions. In the case of FIG. 2b, the intensities detected at the detectors of the apparatus of the ""576 patent shown at FIG. 1 would indicate that the phase-matching condition was met, since the intensities at the two detectors would be equal, while actually the unconverted and converted components are equally and oppositely misaligned from the phase matching direction in terms of intensity. This results in an overall loss of efficiency.
An additional problem with the ""576 patent is that even in the case wherein a spectrally selective detector is used, misalignment of the fundamental beam with respect to the detector is a problem. The reason is that the apparatus would center the converted beam on the detector, which is a problem because the optimum alignment occurs when the converted beam is centered with respect to the fundamental beam. The spectrally selective detector would only detect the converted beam.
It is therefore an object of the invention to provide an automatic phase-matching apparatus and method, wherein fundamental and frequency converted components of a laser beam are phase-matched and aligned together along a phase-matching angle of a nonlinear optical frequency conversion crystal.
In accord with this object, a laser system and method are provided, wherein a laser system includes a laser source and at least one orientationally adjustable and/or temperature-controlled nonlinear optical crystals. First and second position sensitive detectors respectively detect fundamental and higher frequency harmonic beams. A controller receives signals from the first and second detectors indicative of a phase matching error and controls an orientation and/or a temperature of the crystal based on the signals to substantially a phase matching condition.
A method includes generating an original beam at a fundamental frequency, converting the original beam by passing through at least one nonlinear optical crystal to a converted beam including a higher frequency component, detecting a fundamental beam and a higher frequency harmonic beam, and controlling an orientation and/or a temperature of the nonlinear optical crystal based on the detecting of the fundamental beam and the higher frequency harmonic beam to substantially a phase-matching condition.